Fluorescent lamps are widely used for lighting in commercial buildings, residential spaces, as well as on transit buses and in outdoor lighting. Fluorescent lighting provides some advantages, such as improved efficiency, over other lighting options such as incandescent lighting. However, there are several drawbacks. Fluorescent lamps fail under excessive vibration, require a high operating voltage, consume a large amount of power, generally have poor color quality, they cannot be started in cold temperatures or in humid environments, they emit light in 360 degrees about the length of the lamp such that much light is lost in reflection, and they contain mercury, making the lamps difficult to dispose of and hazardous to human health and the environment.
Various solutions offering light emitting diode (LED) based fluorescent tube replacement lamps or other lighting devices have been proposed in U.S. Pat. Nos. 7,049,761, 7,114,830, 7,144,131 and 7,618,157, which are hereby incorporated by reference in their entirety. U.S. Pat. No. 7,049,761 describes fluorescent tube replacement lamps having a row of white LEDs directed towards the area of desired illumination. The LEDs appear as point sources along the length of the lamp, so light is harsh, not uniform or well distributed, and limited to the color quality and consistency of the LED sources. A refracting or scattering cover can be used to diffuse the light for a more uniform appearance, but this either adds significant cost (for a highly efficient diffuser) or loss of lamp efficiency. Furthermore, LEDs generate significant amounts of heat which reduces the lifetime and efficiency of the LED devices. In these lamps, the LED devices are enclosed in a tubular bulb, further increasing the operating temperature due to the large amount of trapped heat. Some lamps incorporate a horizontal heat sink, but such a heat sink, even with fins or grooves, is not very effective. U.S. Pat. No. 7,114,830 describes a fluorescent tube replacement lamp that has LEDs directed towards the area of desired illumination as described above, or directed towards a reflector. The reflector can be used to scatter light out of the lighting unit for a more uniform distribution of the light, however there will still be bright spots. The heat management problems are not addressed. Largely due to heat management issues, these proposed fluorescent tube replacement lamps will have reduced system efficacy, reduced lumen maintenance, problems with color consistency over lifetime, and uncertain reliability. U.S. Pat. No. 7,618,157 proposes a series of blue LEDs exciting a remote phosphor positioned on a plastic cover. Though this patent provides more uniform light, it requires a large amount of phosphor material to manufacture. Phosphor material can be extremely expensive, thus preventing achieving the cost goals required for adoption of this technology. Furthermore, though thermal issues are mitigated with the use of a remote phosphor, thermal management is not optimized and may result in reduced system efficacy, lumen maintenance issues, and uncertain reliability.
Therefore, a need exists for improved systems and methods of illumination. A further need exists for a lighting unit with improved thermal management and efficiency.